Process migration method computer and system

ABSTRACT

Process migration method includes copying first process context indicative of first processing, transmitting process context to second computer, causing first computer to start generation of first execution record, causing second computer to receive process context, determining, from first execution record, whether first processing should be migrated, if it is determined that first processing should postpone being migrated, finishing generation of first execution record, starting generation of second execution record, transmitting first execution record to second computer, reproducing process context, and determining, from second execution record, whether first processing should be migrated, after reproducing of process context is finished in the second computer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromthe prior Japanese Patent Application No. 2002-236432, filed Aug. 14,2002, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process migration method, computerand system for copying, to another computer node, the process currentlyexecuted on a certain computer node, and a computer having such acopying function.

2. Description of the Related Art

A computer system formed of a plurality of computer nodes employs aprocess migration technique for copying, to another computer node, theprocess currently executed on a certain computer node, in order toreduce the load on a certain computer.

Specifically, the process migration technique is a technique forcopying, to another computer node, the state of the process (processcontext) currently executed on a certain computer node, thereby enablinganother computer to continue the execution of the process using thecopied process context. The process context specifically means data(including codes) in a memory space used in the process, register valuesindicative of the state of a central processing unit (CPU), andinformation in a file opened in the process, etc.

In a computer system including a plurality of computer nodes, if a loadis concentrated on a certain computer node, and other computer nodeshave a spare processing capability, the process migration techniqueenables the process executed on the overloaded computer node to becopied to other computer nodes, thereby equalizing the load on thecomputer nodes. In general, if the load on a computer system isequalized, the response time and throughput of the computer system canbe enhanced.

Since recent computer nodes incorporate a large-capacity memory toenhance their processing capability, there is a growing tendency for alarge memory capacity to be needed in a process. Accordingly, the amountof data of a process context is becoming very large. The period requiredfor data communication between computer nodes increases with the amountof communication data. Therefore, the period required for a processcontext to be copied between computer nodes is increasing.

In the conventional process migration technique, to prevent a processcontext from being updated during process migration, the execution ofthe process to be copied is once stopped, as is disclosed in, forexample, Jan. Pat. Appln. KOKAI Publication No. 2000-222376. Therefore,a user utilizing the computer system may think the computer system ishung.

Further, “Preemptable and remote execution facilities for the V-system”(M. Theimer, K. Lantz and D. Cheriton in 10th ACM Symposium on OperatingSystems Principles, pages 2-12, 1985) discloses a technique called“PreCopy”. In this technique, all contents of a process context in amemory space used in a process are copied to a destination computer nodeduring the execution of the process on an originator computer node.After the copying operation, the process is continued and then finishedon the originator computer node. When this process is finished, thecontents of the process context, which have been changed on theoriginator computer node after they are copied to the destinationcomputer node, are also copied to the destination computer node,together with data indicative of the state of the process.

However, if a lot of time is required for copying the contents in thememory space, as stated above, a large amount of contents are changed bythe execution of the process during copying, therefore a lot of time isrequired to copy data indicative of the changed contents to thedestination computer node. As a result, a user may think that thecomputer system is hung for a long time.

As described above, in the conventional process migration technique, theexecution of a process is temporarily stopped. Since, however, there isa tendency for the period required for copying a process context betweencomputer nodes to increase, a user utilizing the computer system maythink that the computer system is hung for a long time.

BRIEF SUMMARY OF THE INVENTION

It is an object of the invention to provide a process migration method,a computer and a system and computer capable of minimizing the period ofprocess stop during process migration, to enhance the responseperformance of the computer for users.

According to a first aspect of the invention, there is provided aprocess migration method for performing a process migration between afirst computer and a second computer which are connected via a network,comprising: (a) copying a process context indicative of first processingwhich the first computer executes; (b) transmitting the process contextto the second computer; (c) causing the first computer to startgeneration of a first execution record concerning the first processingwhen the process context is started to be transmitted; (d) causing thesecond computer to receive the process context, (e) determining, fromthe first execution record, whether or not the first processing shouldbe migrated from the first computer to the second computer;

if it is determined that the first processing should postpone beingmigrated from the first computer to the second computer, (f) finishingthe generation of the first execution record, (g) starting generation ofa second execution record, (h) transmitting the first execution recordto the second computer, the generation of the first execution recordbeing finished, (i) reproducing the process context in the secondcomputer, and (j) determining, from the second execution record, whetheror not the first processing should be migrated from the first computerto the second computer, after reproducing of the process context isfinished in the second computer.

According to a second aspect of the invention, there is provided acomputer serving as a first computer which copies, to a second computervia a network, a process context indicative of processing which thefirst computer executes, comprising; a copy unit configured to copy theprocess context; a first transmission unit configured to transmit theprocess context to the second computer; a first generation unitconfigured to generate a first execution record concerning theprocessing when the process context is started to be transmitted; adetermination unit configured to determine, from the first executionrecord, whether or not the processing should be migrated from the firstcomputer to the second computer;

if the determination unit determines that the processing should postponebeing migrated, a first finishing unit configured to finish thegeneration of the first execution record, a second generation unitconfigured to generate a second execution record, a second transmissionunit to transmit the first execution record to the second computer, thegeneration of the first execution record being finished, and a receivingunit configured to receive, from the second computer, a first messageexpressing that reproduction based on the first execution record isfinished, the determination unit determining, from the second executionrecord, whether or not the processing should be migrated from the firstcomputer to the second computer, after the receiving unit receives thefirst message.

According to a third aspect of the invention, there is provided a systemincluding a first computer and a second computer which are connected viaa network, the first computing executing first processing indicated by aprocess context,

the first computer comprising: a copy unit configured to copy theprocess context; a first transmission unit configured to transmit theprocess context to the second computer; a generation unit configured togenerate a first execution record concerning the processing when theprocess context is started to be transmitted; a determination unitconfigured to determine, from the first execution record, whether or notthe first processing should be migrated from the first computer to thesecond computer;

-   -   if the determination unit determines that the first processing        should postpone being migrated, a first finishing unit        configured to finish the generation of the first execution        record, a second generation unit configured to generate a second        execution record based on the first execution record, a second        transmission unit configured to transmit the first execution        record to the second computer, the generation of the first        execution record being finished, and a first receiving unit        configured to receive, from the second computer, a first message        expressing that reproduction based on the second execution        record is finished,

the second computer comprising: a second receiving unit configured toreceive the process context; a third receiving unit configured toreceive the first execution record; a fourth receiving unit configuredto receive a determination result determined by the determination unit,

-   -   if the determination result indicates that the first processing        should postpone being migrated, a fifth receiving unit        configured to receive the second execution record, a third        transmission unit configured to transmit the first message, and        a fourth transmission unit configured, to the first computer, to        transmit a control signal which controls the determination unit        so as to again determine whether or not the first processing        should be migrated from the first computer to the second        computer, the control signal being transmitted after the        reproduction of the process context is finished in the second        computer.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a block diagram illustrating an embodiment of the structure ofa computer node system to which a process migration method according tothe embodiment is applied;

FIG. 2 is a flowchart useful in explaining processing included in anembodiment of the process migration method and executed by a computernode that copies a process to another computer node;

FIG. 3 is a flowchart useful in explaining processing included in anembodiment of the process migration method and executed by a computernode to which a process is copied; and

FIG. 4 is a view illustrating an execution record data example in anembodiment of the process migration method.

DETAILED DESCRIPTION OF THE INVENTION

A process migration method, a compute and a system according to anembodiment of the invention will be described with reference to theaccompanying drawings.

Firstly, the outline of the embodiment will be described.

The process migration method of an embodiment is suitable for copying aprocess from a certain computer node (hereinafter referred to as an“originator computer node”) to another computer node (hereinafterreferred to as a “destination computer node”) connected thereto via anetwork.

For facilitating the description, in the embodiment, only two computernodes, i.e., an originator computer node and destination computer node,are connected to a network. Of course, three or more computer nodes maybe connected.

The originator computer node is provided with anexecution-record-recording device for recording the execution results(records) of a to-be-copied process. On the other hand, the destinationcomputer node is provided with an execution reproduction device forreproducing the execution results recorded by theexecution-record-recording device. The execution-record-recording deviceand execution-record reproduction device are used to make a copiedprocess on the destination computer node accord to an original processon the originator computer node.

At the start of process migration, on the originator computer node, acopy of the contents in a memory space used in a to-be-copied process isgenerated. After the generation of the copy, the execution results ofthe to-be-copied process (i.e., the original process) are sequentiallyrecorded in the execution-record-recording device, while the executionof the to-be-copied (original) process is continued.

The execution results to be recorded include, for example, an argumentand its results, etc. used for calling an operating system or library.In the embodiment, to estimate the period required for reproduction inthe destination computer based on the execution results, the rate of useof the CPU in the to-be-copied (original) process is also recorded as anexecution result.

The copy of the contents in the memory space used in the to-be-copiedprocess, performed on the originator computer node, is transferred tothe destination computer node via the network. On the destinationcomputer node, the memory space is reproduced using the copy. Duringthis operation, the execution of the to-be-copied (original) process iscontinued on the originator computer node, therefore a user utilizingthe computer system does not think the computer system is hung.

After the transfer of the copy of the contents in the memory space usedin the to-be-copied process finishes, the originator computer nodeestimates, from the execution results recorded in theexecution-record-recording device and the rate of use of the CPU, theperiod required for reproduction of the execution results on thedestination computer node. The estimated required period determineswhether the following process (A) or (B) should be executed:

(A) If the period required for the reproduction based on the executionresults, estimated by the originator computer node, is enough to beshort, copying of the to-be-copied process is performed on thedestination computer node. To this end, the execution results are copiedto the destination computer node, and at the same, the to-be-copiedprocess currently executed on the originator computer node is cancelled.After that, the originator computer node informs the destinationcomputer node of the fact that the original process has been cancelled.

(B) If the period required for the reproduction based on the executionresults, estimated by the originator computer node, is still long, theoriginator computer node continues the process. To this end, theexecution records are copied to the destination computer node, and theexecution of the process is continued on the originator computer node,thereby updating the execution results (records).

The execution reproduction device of the destination computer nodereproduces the execution records on the basis of the execution recordsrecorded in the originator computer node, thereby causing the state ofthe copy of the process to accord to the updated, recorded state of theoriginal process.

In the reproduction operation, the call-up results of the operatingsystem or library recorded as execution records are used instead ofactually calling up the operating system or library. Therefore, theperiod required for calling up them is not needed. Accordingly, the CPUcan be used only for reproduction, and the period required forreproduction is (the whole execution period X the rate of use of theCPU). In other words, reproduction of execution records can be performedat a higher speed than the actual execution of a process.

Accordingly, in the case (B), it is considered that the amount ofexecution result data newly recorded on the originator computer nodeduring reproduction on the destination computer node is smaller thanthat of previous execution result data. Therefore, except for anexceptional case described later, the amount of execution result databecomes smaller when recording and reproduction of execution result datais repeated, and the period required for reproduction of executionresult data can be accordingly minimized.

If the rate of use of the CPU in the original process is exceptionallynear 100%, there is almost no time difference between the reproductionof execution result data and usual processing, therefore the periodrequired for the reproduction of execution result data cannotsubstantially be reduced even if recording and reproduction of executionresult data is repeated. In this case, instead of the repetitionprocessing, the operation is shifted to a process on the destinationcomputer node.

The destination computer node executes a to-be-copied process on thebasis of received execution records. After all execution records havebeen reproduced, the copied state of the process is the same as thestate of the original process on the originator computer nodecorresponding to execution records of the final stage.

If at this time, the original process is already cancelled on theoriginator computer node, the copied state of the to-be-copied processon the destination computer node is an updated state.

When the originator computer node informs the destination computer nodethat the original process has been cancelled, the remaining processingof the original process is performed on the destination computer node,using the copied process. This is the termination of process migration.

Further, when the original process is continued on the originatorcomputer node, the destination computer node again estimates the periodrequired for reproducing the execution records, on the basis of theexecution records and the rate of use of the CPU that are newly recordedon the destination computer node, and repeats the transfer andreproduction processing of execution records.

The above-described processing can reduce the period required forreproducing execution records, which is equal to the period required forcopying a process from an originator computer node to a destinationcomputer node. As a result, the temporary stop period of a process canbe further reduced.

Referring to FIG. 1, the structure of the embodiment will be described.

FIG. 1 shows a computer system comprising computer nodes 101 and 102connected via a communication line 103.

The computer node 101 comprises a process-copying unit 104 for copying ato-be-copied (original) process 106 to the computer node 102 as adestination node, and a communication unit 105 for communication withthe computer node 102.

The process-copying unit 104 is formed of a process-context-copying unit107 and execution-record-recording unit 109. The process-context-copyingunit 107 manages a process context storage 108, and theexecution-record-recording unit 109 manages execution records 110.

The computer node 102 comprises a process-receiving unit 111 forreceiving a copy of the original process 106 from the computer node 101and generating a copied process 113, and a communication unit 112 forcommunication with the computer node 101.

The process-receiving unit 111 is formed of a process context-receivingunit 114 and execution reproduction unit 116. The processcontext-receiving unit 114 manages a process context storage 115, andthe execution reproduction unit 116 manages execution records 117.

Although in the embodiment, one computer node employs either a processcopy unit or process-receiving unit, it may employ both the units. Inthe latter case, process migration can be performed bidirectionally.

(Process Migration at Originator Node) Referring to the flowchart ofFIG. 2, process migration processing performed by the computer node 101as the originator will be described.

(Transfer of Process Context) The steps 201-206 of the flowchart of FIG.2 illustrate the processing performed by the process-context-copyingunit 107 to transfer a copy of the contents in a memory space used inthe original process 106.

Firstly, the process-context-copying unit 107 temporarily stops theexecution of the original process 106 (step 201), then generates a copyof information used in the original process 106, such as CPU registervalues, file information, etc. and a copy of the contents in a memoryspace used in the original process 106, and stores them into the processcontext storage 108 (step 202).

Subsequently, the process-context-copying unit 107 instructs theexecution-record-recording unit 109 to start recording of the executionresults of the original process 106 (step 203), and resumes theexecution of the original process 106 (step 204).

After that, the process-context-copying unit 107 transfers, to thecomputer node 102, the copy of the information such as CPU registervalues, file information, etc. and the copy of the contents in thememory space stored in the process context storage 108 (step 205).

Thereafter, the process-context-copying unit 107 waits for thecompletion of transfer of the contents (step 206).

(Recording of Execution Records, Shifting of Processing) Referring nowto steps 207 et seq., recording of execution records by theexecution-record-recording unit 109 and shifting of processing will bedescribed.

After receiving an instruction to start recording at the step 203, theexecution-record-recording unit 109 records, as execution records 110,the execution records of the original process 106.

Upon receiving a message that reproduction is started, theexecution-record-recording unit 109 temporarily stops the execution ofthe original process 106 (step 207), and estimates the period requiredfor reproducing the accumulated execution records 110 (step 208). In theembodiment, the estimation of the period required for reproduction isperformed on the basis of execution results recorded as executionrecords 110, the rate of use of the CPU, the amount of the executionrecords 110, etc. If simple estimation is preferred, the required periodmay be determined only from the data amount of the execution records110, without considering the contents of the records 110.

The estimated period is compared with a predetermined reference period(step 209). If the former is shorter than the predetermined referenceperiod, the original process 106 is cancelled (step 210), therebysupplying the execution reproduction unit 116 with a message that theexecution records 110 and the original process 106 have been cancelled(step 211).

On the other hand, if the required period is equal to or longer than thereference period, the execution-record-recording unit 109 finishesrecording of the present execution records of the original process 106,then starts recording of new execution records (step 212), resumes theexecution of the original process 106 (step 213), supplies the computernode 112 with information and previous execution records 110 (i.e., theexecution records obtained at the step 212) (step 214), and waits forthe completion of reproduction (step 215).

(Process Migration at Destination Node) Referring to the flowchart ofFIG. 3, a description will be given of process migration performed atthe computer node 102 as the destination node.

(Transfer of Process Context) The steps 301-303 of FIG. 3 illustrate theprocessing of the process context-receiving unit 114 to receive a copyof the contents in a memory space used in the original process 106.

The process context-receiving unit 114 receives the information such asCPU register values, file information, etc. and the contents in thememory space transmitted from the computer node 101 via thecommunication unit 112, stores them into the process context storage 115(step 301), and generates a copied process 113 (=restoration of theoriginal process 106) using the information such as CPU register values,file information, etc. and the contents in the memory space (step 302).

After completion of restoration of the process, the processcontext-receiving unit 114 informs the process-context-copying unit 107that the transfer of the information such as CPU register values, fileinformation, etc. and the contents in the memory space has been finished(step 303).

(Reproduction of Execution Records) Steps 304 et seq. of FIG. 3illustrate reproduction of execution records in the executionreproduction unit 116.

The execution reproduction unit 116 receives execution records from theexecution-record-recording unit 109, and accumulates them as executionrecords 117 (step 304). After that, the unit 116 starts reproduction ofthe copied process 113 using, for example, the received executionrecords (step 305).

After finishing reproduction of all execution records (step 306), theunit 116 confirms whether or not the original process 106 is cancelled(step 307).

If the original process 106 is cancelled, this means that theto-be-copied process has completely been copied, therefore the executionof the copied process 113 is started (step 308).

If the original process 106 is not cancelled, the reproduction of thecopied process 113 is temporarily stopped (step 309), thereby informingthe execution-record-recording unit 109 that reproduction is finished(step 310).

As described above, continuous execution of an original process on anoriginator computer node during process migration enables a userutilizing the computer to think that the computer is operating evenduring process migration. Further, if a lot of time is required forreproduction of execution records, the period of stopping the processcan be further reduced by repeatedly performing the recording andreproduction of execution records.

(Examples of Execution Records) Examples of execution records 110 areshown in FIG. 4.

The execution records 110 comprise the contents 402 of requests andexecution results 403 in a process.

For example, data “2002/6/4 20:30” is recorded as a result 404 obtainedby the execution of a request “Acquire Present Time”. Accordingly, evenif, in a copied process, the data “Acquire Present Time” is reproducedafter “2002/6/4 20:30”, for example, at “2002/6/4 20:33”, the data“2002/6/4 20:30” is correctly reproduced. Thus, the states of theoriginal process and copied process can be made identical.

Further, read data is recorded as a result 405 obtained by the executionof a request “Read Data from File”. In general, files are recorded in adevice, such as an HDD (hard disk drive), which is of a much sloweraccess speed than, for example, a memory. In light of this, to reproduceexecution records, the execution records are read from a memory, whichdiffers from a usual process where execution records are read from anHDD. Accordingly, reproduction can be executed at a higher speed thanusual processing.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

1. A process migration method for performing a process migration between a first computer and a second computer which are connected via a network, comprising: (a) copying a process context indicative of first processing which the first computer executes; (b) transmitting the process context to the second computer; (c) causing the first computer to start generation of a first execution record concerning the first processing when the process context is started to be transmitted; (d) causing the second computer to receive the process context; (e) determining, from the first execution record, whether or not the first processing should be migrated from the first computer to the second computer; if it is determined that the first processing should not be migrated from the first computer to the second computer, (f) finishing the generation of the first execution record; (g) starting generation of a second execution record; (h) transmitting the first execution record to the second computer, the generation of the first execution record being finished; (i) reproducing the process context in the second computer; and (j) determining, from the second execution record, whether or not the first processing should be migrated from the first computer to the second computer, after reproducing of the process context is finished in the second computer, wherein if it is determined that the first processing should be migrated, (k) finishing execution of the first processing; (l) finishing generation of the first execution record; (m) transmitting the first execution record to the second computer; (n) reproducing the process context in the second computer; and (o) causing the second computer to start execution of second processing indicated by the process context after the process context is reproduced.
 2. The migration method according to claim 1, wherein the determining (e) includes estimating a period in which the second computer reproduces the first execution record, and determining that the first processing should be migrated if the period is shorter than a certain period.
 3. The migration method according to claim 2, wherein the determining (e) further includes computing an average value of rate of use of a CPU which executes the first processing, the CPU being included in the first computer, and determining that the first processing should be migrated if the average value exceeds a threshold value.
 4. The migration method according to claim 1, wherein the determining (e) includes computing a data amount of the first execution record, and determining that the first processing should be migrated if the data amount is smaller than a data amount.
 5. The migration method according to claim 4, wherein the determining (e) further includes computing an average value of rate of use of a CPU which executes the first processing, the CPU being included in the first computer, and determining that the first processing should be migrated if the average value exceeds a threshold value.
 6. The migration method according to claim 1, wherein the copying (a) includes generating the process context which includes at least one of data on a memory space and a value of a register incorporated in a CPU included in the first computer.
 7. A computer serving as a first computer which copies, to a second computer via a network, a process context indicative of processing which the first computer executes, comprising; a copy unit configured to copy the process context; a first transmission unit configured to transmit the process context to the second computer; a first generation unit configured to generate a first execution record concerning the processing when the process context is started to be transmitted; a determination unit configured to determine, from the first execution record, whether or not the processing should be migrated from the first computer to the second computer; a first finishing unit configured to finish the generation of the first execution record, if the determination unit determines that the processing should not be migrated; a second generation unit configured to generate a second execution record, if the determination unit determines that the processing should not be migrated; a second transmission unit configured to transmit the first execution record to the second computer, the generation of the first execution record being finished, if the determination unit determines that the processing should not be migrated; and a receiving unit configured to receive, from the second computer, a first message expressing that reproduction based on the first execution record is finished, if the determination unit determines that the processing should not be migrated, the determination unit determining, from the second execution record, whether or not the processing should be migrated from the first computer to the second computer, after the receiving unit receives the first message.
 8. The computer according to claim 7, wherein a second finishing unit configured to finish execution of the processing, if the determination unit determines that the processing should be migrated; a third finishing unit configured to finish generation of the first execution record, if the determination unit determines that the processing should be migrated; a third transmission unit configured to transmit the first execution record to the second computer, if the determination unit determines that the processing should be migrated; and a fourth transmission unit configured to transmit, to the second computer, a second message expressing that the execution and the generation are finished, if the determination unit determines that the processing should be migrated.
 9. The computer according to claim 7, wherein the determination unit estimates a period in which the second computer reproduces the first execution record, and determines that the processing should be migrated if the period is shorter than a certain period.
 10. The computer according to claim 9, wherein the determination unit computes an average value of rate of use of a CPU which executes the processing, the CPU being included in the computer, and determines that the processing should be migrated if the average value exceeds a threshold value.
 11. The computer according to claim 7, wherein the determination unit computes a data amount of the first execution record, and determines that the processing should be migrated if the data amount is smaller than a data amount.
 12. The computer according to claim 11, wherein the determination unit computes an average value of rate of use of a CPU which executes the processing, the CPU being included in the computer, and determines that the processing should be migrated if the average value exceeds a threshold value.
 13. The computer according to claim 7, wherein the copy unit generates the process context which includes at least one of data on a memory space and a value of a register incorporated in a CPU included in the computer.
 14. A system including a first computer and a second computer which are connected via a network, the first computing executing first processing indicated by a process context, the first computer comprising: a copy unit configured to copy the process context; a first transmission unit configured to transmit the process context to the second computer; a generation unit configured to generate a first execution record concerning the processing when the process context is started to be transmitted; a determination unit configured to determine, from the first execution record, whether or not the first processing should be migrated from the first computer to the second computer; a first finishing unit configured to finish the generation of the first execution record, if the determination unit determines that the first processing should not be migrated; a second generation unit configured to generate a second execution record based on the first execution record, if the determination unit determines that the first processing should not be migrated; a second transmission unit configured to transmit the first execution record to the second computer, the generation of the first execution record being finished, if the determination unit determines that the first processing should not be migrated; and a first receiving unit configured to receive, from the second computer, a first message expressing that reproduction based on the second execution record is finished, if the determination unit determines that the first processing should not be migrated; the second computer comprising: a second receiving unit configured to receive the process context; a third receiving unit configured to receive the first execution record; a fourth receiving unit configured to receive a determination result determined by the determination unit, a fifth receiving unit configured to receive the second execution record, if the determination result indicates that the first processing should not be migrated; a third transmission unit configured to transmit the first message, if the determination result indicates that the first processing should not be migrated; and a fourth transmission unit configured to transmit, to the first computer, a control signal which controls the determination unit so as to again determine whether or not the first processing should be migrated from the first computer to the second computer, the control signal being transmitted after the reproduction of the process context is finished in the second computer, if the determination result indicates that the first processing should not be migrated.
 15. The system according to claim 14, wherein the first computer further comprises: a second finishing unit configured to finish execution of the first processing, if the determination unit determines that the first processing should be migrated; a third finishing unit configured to finish generation of the first execution record, if the determination unit determines that the first processing should be migrated; a fifth transmission unit configured to transmit the first execution record to the second computer, if the determination unit determines that the first processing should be migrated; and a sixth transmission unit configured to transmit, to the second computer, a second message expressing that the execution and the generation are finished, if the determination unit determines that the first processing should be migrated.
 16. The system according to claim 14, wherein the second computer further comprises: a sixth receiving unit configured to receive the second execution record, if the determination result indicates that the first processing should be migrated; a seventh receiving unit configured to receive the second message, if the determination result indicates that the first processing should be migrated; and a start unit configured to start execution of second processing indicated by the process context after reproduction of the process context is finished, if the determination result indicates that the first processing should be migrated.
 17. The system according to claim 14, wherein the determination unit estimates a period in which the second computer reproduces the first execution record, and determines that the first processing should be migrated if the period is shorter than a certain period.
 18. The system according to claim 17, wherein the determination unit computes an average value of rate of use of a CPU in the first computer which executes the first processing, and determines that the first processing should be migrated if the average value exceeds a threshold value.
 19. The system according to claim 14, wherein the determination unit computes a data amount of the first execution record, and determines that the first processing should be migrated if the data amount is smaller than a data amount.
 20. The system according to claim 19, wherein the determination unit computes an average value of rate of use of a CPU in the first computer which executes the first processing, and determines that the first processing should be migrated if the average value exceeds a threshold value.
 21. The system according to claim 14, wherein the copy unit generates the process context which includes at least one of data on a memory space and a value of a register incorporated in a CPU included in the first computer. 